The lost soul is a flaming, floating skull with horns, first introduced in Doom's second episode, The Shores of Hell. Lost souls use themselves as weapons, charging towards the player with open jaws. Given its tendency to charge suddenly, its quickness to retaliate upon being attacked, and its ability to crowd the player, a lost soul can be a considerable annoyance. Doom II finds a new use for the lost soul by employing it as the projectile attack of the game's new pain elemental monster. The lost soul is officially described as such:

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Arguably one of the most frustrating enemies in the Doom games, the lost soul floats about at a leisurely pace until it decides to attack, at which point it launches itself towards its target at high speed with a hissing breath sound, often the first audible warning of their presence. Unlike all other monsters, the lost soul is completely silent when alerted, giving it an element of stealth. If a lost soul's attack misses, it will continue careening through the air until it strikes a wall or other object. A lost soul's charge can be intercepted by a projectile or hitscan attack; when this happens, the impact negates the lost soul's momentum without interrupting the charge and as a result the monster will slowly drift, still considered as being in a charge but unable to perform any other action until it collides with something. On maps with large open areas, like E3M6: Mt. Erebus, a lost soul that missed or was pushed back can travel for a long while before being able to become active again.

It is uncommon to find a lost soul on its own — they are more often found in groups. Since lost souls have 100 hit points each, a group of lost souls can drain precious ammo, especially during pain elemental battles.

The lost soul is one of two monsters that do not leave a corpse (the other being the pain elemental); it simply explodes. Because of this, the lost soul does not normally respawn when the Nightmare! skill level is chosen or the -respawnparameter is used. The lost soul will however leave a small pile of gibs if it is crushed while dying, in a similar manner to the pain elemental. Regardless, it is not possible for an arch-vile to resurrect a lost soul even if it is crushed while it dies, as it does not have the necessary resurrection frames.

Note that the lost soul sprite always remains fully bright in the same way projectiles and lamps are lit up, to represent the light emitted by their flames.

In Doom II and Doom v1.666, lost souls were modified to no longer affect the player's kill score, so it is possible to achieve 100% kills without destroying any lost souls found in a level. This was undoubtedly due to the addition of the pain elemental. However, in console ports such as the PlayStation and Saturn versions of Doom, they are still considered part of the kill score, easily pushing the kill ratio past 100%. This is because of their inheritance from the Atari Jaguar source, which predated Doom II's development. This was corrected for Doom 64.

The player can stop a lost soul's charge by shooting it while it is attacking. Because of their 100% pain chance, it is often safer to use the pistol against them, rather than the slower-fire shotgun, so the lost soul will not have enough time to retaliate. Similarly, a chaingun can be useful against a crowd of lost souls, especially due to the Lost soul charging backwards effect — if shot when charging, a lost soul becomes essentially helpless until it hits something. Also, lost souls will stop when they cross an object in their charge path, such as a shotgun or a medikit, which stops them short. Using rockets against groups of lost souls is strongly discouraged, as there is a considerable chance that a lost soul will suddenly fly towards the player, causing one of the rockets to explode close enough to cause blast damage to the foolish marine. It is best to never use the rocket launcher at all in areas where lost souls are present.

One unique characteristic of this monster type is that after it attacks, it does not retain its target, and will instead return to pursuit of the player. This makes lost souls unreliable for provoking and attempting to sustain monster infighting. However, a lost soul's high pain chance and retaliation behavior does make infighting between lost souls more likely to sustain itself, with the two souls repeatedly attacking each other while climbing higher toward the ceiling.

In the available pre-release versions of Doom, lost souls are portrayed as a bleached, silver demon skull with an open jaw. These lost souls are non-flaming and use a "psychic" attack, facing the victim and flashing for a moment, causing instantaneous and unavoidable damage. As true flying monsters had not been added to the engine yet, these beta lost souls cannot actually fly. Their sprites are instead offset a constant distance from the floor via their patch offsets. These early lost souls leave behind a pile of floating bones when they are killed. The invulnerability powerup uses modified art assets from this version of the monster.

There exists a thing type in the final version of the game, named MT_MISC65 (called "Dead Lost Soul" in DeHackEd), which produces the last few death frames of the lost soul. This dead lost soul was evidently left over from earlier versions of the game where the monster did leave a corpse, such as in the press release beta. This thing would have served the same function as the other decorative corpses (the imp and the zombieman corpses, for example). Some early mappers used dead lost soul things to crash the Doom executable when the shareware version allowed the use of PWADs, to encourage people to buy the game. This technique is also used by the map generator SLIGE.

The beta lost soul is recreated to a high degree of accuracy in the MBFsource port, which has a beta emulation mode. Programmer Lee Killough used frame-by-frame advancement to estimate the tic counts of its states, and statistical analysis of damage done in order to code its attack. The Eternity Engine also has a version of the monster, though with deliberately less accuracy in order to make it behave consistently with other monsters in the game engine.

The sprite graphics for the lost soul were revised between v1.2 and Doom II, with significant changes to the structure of the skull, particularly around the eyes. According to John Romero, these changes were made by Adrian Carmack for purely aesthetic reasons.[1] Ports based on the Atari Jaguar version retained the older sprites, up to and including the Sony PlayStation games. The older graphics also continue to be displayed on modern versions of the Doom II retail packaging's reverse panel.

This table assumes that all calls to P_Random for damage, pain chance, and blood splats are consecutive. In real play, this is never the case: counterattacks and AI pathfinding must be handled, and of course the map may contain additional moving monsters and other randomized phenomena (such as flickering lights). Any resulting errors are probably toward the single-shot average, as they introduce noise into the correlation between the indices of "consecutive" calls.

Assumes that direct hits are possible, which does not occur in any stock map.

In Doom RPG, the lost soul appears as a class of monster. There are three variations, identified by color:

Phantom (green)

Lost soul (normal colors)

Nightmare (blue)

A lost soul can move three panels in a single turn instead of one, and will attack automatically once it is in an adjacent panel to the player, even if it has already moved. It attacks three times consecutively and can be damaged by the fire extinguisher.

This variation of the lost soul is slightly transparent and has an opened skull with fire emerging only from its top. It is much more aggressive than its PC counterpart, with a much faster recovery time from its attack. This makes the lost soul a high-tier threat. To make up for this, however, it is also physically weaker than its PC counterpart and can be felled with a single shotgun blast.